Lithium-ion secondary batteries with high energy densities have been used as power sources for portable electronics such as a mobile phone and a notebook computer.
An electrode member of a lithium-ion secondary battery includes a positive electrode plate, a separator, and a negative electrode plate. Regarding a positive electrode material, an aluminum alloy foil has been used as a support, having excellent electrical conductivity and less heat generation without affecting electrical efficiency of a secondary battery. An active material having a lithium-containing metal oxide such as LiCoO2 as a chief component is applied on a surface of the aluminum alloy foil. Its production process includes: applying an active material with a thickness of about 100 μm on both sides of an aluminum alloy foil with a thickness of about 20 μm; and providing a heat treatment to remove a solvent therefrom (hereinafter referred to as drying process). Further, in order to increase the density of the active material, compression forming is performed with a pressing machine (hereinafter, this step of compression forming performed with a pressing machine is referred to as press working). The positive electrode plate as so manufactured, a separator, and a negative electrode plate are stacked, and then the resulting stack is wound. After a shaping process is performed so as to encase the stack, it is encased.
An aluminum alloy foil used for an electrode material of a lithium-ion secondary battery has several problems that cuts occur during application of an active material and that ruptures occur at a bending portion during winding. Thus, a higher strength is required. In particular, heat treatment is carried out at about 100 to 180° C. in the drying process. Accordingly, when the strength after the drying process is low, the aluminum alloy foil is easily deformed during press working. This leads to decrease in adhesion between the active material and the aluminum alloy foil. Besides, a rupture is likely to occur during a slitting process. When the adhesion between the active material and a surface of the aluminum alloy foil decreases, their detachment is facilitated during repeated operation of discharge and charge. Unfortunately, this causes its battery capacity to decrease.
Recently, a high electrical conductivity has been also required for an aluminum alloy foil used for an electrode material of a lithium-ion secondary battery. What is meant by the electrical conductivity refers to physical property indicating how easily electricity is conducted in a substance. The higher the electrical conductivity is, the more easily the electricity is conducted. Lithium-ion secondary batteries used for automobiles and/or electric tools necessitate a higher output characteristic than lithium-ion secondary batteries used for consumer-use mobile phones and/or notebook computers. When a large current flows, a lower electrical conductivity causes internal resistance of a battery to increase. Consequently, this reduces its output voltage. Accordingly, the aluminum alloy foils used for the lithium-ion secondary batteries require high strength for both the foils before and after the drying process, and high electrical conductivity.
Aluminum alloy foils for lithium ion secondary batteries are generally manufactured by semi-continuous casting. In the semi-continuous casting, an ingot is obtained by casting aluminum alloy molten metal. Then, the obtained ingot is subject to hot rolling and cold rolling to give an aluminum sheet (foil material) having a thickness of about 0.2 to 0.6 mm, followed by foil rolling to give the aluminum alloy foil having a thickness of about 6 to 30 μm. Here, homogenization treatment of the ingot and intermediate annealing in the midst of the cold rolling are also generally performed as necessary.
In the continuous casting, a cast sheet is obtained by casting and rolling the aluminum alloy molten metal continuously. Therefore, in the continuous casting, the homogenization treatment of the ingot and the hot rolling, which are the essential steps in the semi-continuous casting, can be omitted. Therefore, yield and energy efficiency can be improved, achieving low manufacturing cost. The typical continuous casting includes twin roll continuous casting and twin belt continuous casting. The cooling speed of the molten metal in these continuous casting is faster than that in the semi-continuous casting. Therefore, the elements added to the aluminum are forced to form solid solution in a supersaturated manner, thereby precipitating uniform and fine crystals of intermetallic compounds. In addition, the cast sheet after the continuous casting can obtain high electrical conductivity by performing heating treatment in the midst of the cold rolling, thereby allowing the supersaturated solid solution of Fe to precipitate. As a result, when compared with the aluminum alloy foil manufactured by the semi-continuous casting, the aluminum alloy foil manufactured by the continuous casting has higher strength.
In general, cast sheet after continuous casting is performed with heat treatment in the middle of cold rollings, in order to improve the rolling property. When heat treatment is performed, Fe dissolved in a supersaturated manner partially precipitates, resulting in decrease of solid solution content of Fe. However, since the finely precipitated intermetallic compound contribute to the dispersion strenghthening, the aluminum alloy foil manufactured by the continuous casting has higher strength than the aluminum alloy foil manufactured by the semi-continuous casting. Here, when the heat treatment is omitted, an aluminum alloy foil with higher strength and higher strength after the drying process can be obtained, due to the high the solid solution content of Fe dissolved in a supersaturated manner and the finely precipitated intermetallic compound. In addition, by omitting the heat treatment performed with the cast sheet after the continuous casting, cost for manufacture can be reduced when compared with the aluminum alloy foil manufactured by the continuous casting which performs heat treatment after the cold rolling.
Patent Literature 1 discloses an aluminum alloy material having excellent corrosion resistance, containing only Fe and having an intermetallic compound with a maximum length of 2.0 μm or longer and aspect ratio of 3 or more distributed by 30/10000 (μm)2. However, since there is no limitation with respect to the amount of Si, the intermetallic compound which precipitate during the continuous casting is likely to become large, which results in decrease in the number of uniform and fine intermetallic compound that contribute to improvement in strength. Although Patent Literature 1 is silent from any particular disclosure with respect to the electrode material, if an aluminum foil is used as the aluminum alloy foil for lithium ion secondary battery, the strength after heat treatment, which simulates a drying process, would be low. This strength is insufficient since the adhesion between the active material and the aluminum alloy foil decreases and the aluminum alloy foil becomes prone to ruptures during a slitting process because the aluminum alloy foil is easily deformed during press working.
Patent Literature 2 discloses an aluminum alloy foil for electrode current collectors used in lithium ion battery, which is manufactured by the semi-continuous casting and has a strength of 160 MPa or higher. However, the strength after heat treatment, which simulates a drying process, would be low. This strength is insufficient since the adhesion between the active material and the aluminum alloy foil decreases and the aluminum alloy foil becomes prone to ruptures during a slitting process because the aluminum alloy foil is easily deformed during press working.
Patent Literature 3 discloses a manufacturing method for an aluminum alloy foil, which includes: obtaining a cast sheet with a thickness of 25 mm or less by continuous casting, then performing cold rolling at the rate of 30% or higher, followed by performing heat treatment at a temperature of 400° C. or higher, and then performing intermediate annealing at a temperature between 250 and 450° C. Although the aluminum alloy foil obtained from this aluminum alloy foil sheet has superior rolling property since heat treatment is performed, the precipitation of each elements that were dissolved in a supersaturated manner would lead to insufficient strength. Such strength is insufficient since the adhesion between the active material and the aluminum alloy foil decreases and the aluminum alloy foil becomes prone to ruptures during a slitting process because the aluminum alloy foil is easily deformed during press working.